Nitric oxide (NO) synthesized from arginine by endothelial nitric oxide synthase (eNOS) is a potent vasodilator and a critical modulator of blood flow and blood pressure. In addition, it mediates vasoprotective actions through inhibiting smooth muscle proliferation, platelet aggregation, and leukocyte adhesion (Bredt, D. S. and Snyder, S. H. Annu. Rev. Biochem., 1994, 63:175-195; Gow, A. J. and Ischiropoulos, H. J. Cell. Physiol., 2001, 187:277-282; Vallance, P. and Chan, N. Heart, 2001, 85:342-50). Under pathophysiological conditions associated with endothelial dysfunction, such as heart failure (Sharma, R. and Davidoff, M. N. Congest. Heart Fail., 2002, 8:165-172), hypertension, hypercholesterolemia, atherosclerosis (Maxwell, A. J. Nitric Oxide, 2002, 6:101-124), and diabetes (Goligorsky, M. S. and Gross, S. S. Drug News Perspect., 2001, 14:133-142), the ability to produce NO seems to be impaired. One suggested reason for this impairment has been the reduced availability of the substrate arginine, despite saturating levels of intracellular and extracellular arginine.
The present inventors have previously shown that, under normal conditions, the essential arginine available for NO production is derived from the recycling of citrulline back to arginine, catalyzed by two enzymes, argininosuccinate synthase (AS) and argininosuccinate lyase (AL) (Flam, B. R. et al. Nitric Oxide, 2001, 5:187-197; Pendleton, L. C. et al. J. Biol. Chem., 2002, 277:25363-25369; Goodwin, B. L. et al. J. Biol. Chem., 2004, 279:18353-18360). Although these two enzymes have been studied extensively in the liver where they participate in the urea cycle (Morris, S. M., Jr. Annu. Rev. Nutr., 1992, 12:81-101), it was not until the discovery of NO that their function in non-hepatic tissue was clarified.
Because AS catalyzes the rate-limiting step in the citrulline-NO cycle (Xie, L. and Gross, S. S. J. Biol. Chem., 1997, 272:16624-16630), the present inventors' initial studies have focused on the molecular basis for the functional role of endothelial AS. Endothelial and hepatic AS appear to have the same primary structure (Pendleton, L. C. et al. J. Biol. Chem., 2002, 277:25363-25369; Freytag, S. O. et al. J. Biol. Chem., 1984, 259:3160-31662), but differ in cellular location and level of expression (Flam, B. R. et al. Nitric Oxide, 2001, 5:187-197; Pendleton, L. C. et al. J. Biol. Chem., 2002, 277:25363-25369). Hepatic urea cycle AS and AL are associated with the mitochondria (Cohen, N. S. and Kuda, A. J Cell. Biochem., 1996, 60:334-340), while in endothelial cells, AS and AL co-localize with eNOS in caveolae (Flam, B. R. et al. Nitric Oxide, 2001, 5:187-197).
AS expression in liver also differs from AS expression in endothelial cells as demonstrated by the diversity of 5′-UTR AS mRNA species in endothelial cells (Pendleton, L. C. et al. J. Biol. Chem., 2002, 277:25363-25369). Three transcription initiation sites identified in endothelial cells result in overlapping 5′-UTR regions of 92, 66 and 43 nucleotides (nt). The longer forms make up ˜7% of the total AS message, with the shortest 43 nt 5′-UTR AS mRNA being the predominant species in endothelial cells, and the only detectable form found in liver. The extended 92 and 66 nt 5′-UTR AS mRNAs contain an out-of-frame, upstream overlapping ORF that is terminated by a stop codon 70 nt past the in-frame start codon for the downstream ORF encoding AS. Previously, the present inventors reported that in vitro translation of AS mRNA containing the extended 5′-UTRs was suppressed compared to the shortest and most predominant 43 nt 5′-UTR AS mRNA species (Pendleton, L. C. et al. J. Biol. Chem., 2002, 277:25363-25369). Moreover, it was also showed that the translational efficiency of the extended 5′-UTR AS mRNA species was restored to short form levels when the uAUG was mutated to AAG, thus eliminating the uORF (Pendleton, L. C. et al. J. Biol. Chem., 2002, 277:25363-25369).
Upstream ORFs can act as cis-acting factors affecting the translation of a downstream ORF in a variety of ways (Morris, D. R. and Geballe, A. P. Mol. Cell. Biol., 2000, 20:8635-8642). In higher eukaryotes, initiation of translation generally occurs at the first AUG that resides in a favorable context. When the first AUG context is suboptimal, a portion of the scanning ribosomes may continue past the first AUG and initiate translation downstream at subsequent AUGs via leaky scanning (Kozak, M. Gene, 1999, 234:187-208). In this context, a significant number of eukaryotic mRNAs have now been reported that contain one or more upstream ORFs shown to affect the translational efficiency of the main, downstream ORF (Morris, D. R. and Geballe, A. P. Mol. Cell. Biol., 2000, 20:8635-8642). Depending on conditions such as intercistronic length and secondary structure, scanning ribosomes, upon initiation at the uAUG, can either translate the uORF and reinitiate downstream or stall on the mRNA during elongation, thus preventing initiation at other sites (Morris, D. R. and Geballe, A. P. Mol. Cell. Biol., 2000, 20:8635-8642). In other cases, partial translation of the nascent peptide prevents downstream re-initiation by interaction of the peptide with a protein or RNA in the ribosome preventing termination from proceeding efficiently (Gaba, A. et al. Embo. J., 2001, 20:6453-6463). Another less common event is for the uORF to be translated and for the peptide product to affect translation of the downstream cistron via a trans mechanism (Parola, A. L. and Kobilka, B. K. J. Biol. Chem., 1994, 269:4497-4505).
RNA interference (RNAi) is a polynucleotide sequence-specific, post-transcriptional gene silencing mechanism effected by double-stranded RNA that results in degradation of a specific messenger RNA (mRNA), thereby reducing the expression of a desired target polypeptide encoded by the mRNA (see, e.g., WO 99/32619; WO 01/75164; U.S. Pat. No. 6,506,559; Fire et al., Nature 391:806-11 (1998); Sharp, Genes Dev. 13:139-41 (1999); Elbashir et al. Nature 411:494-98 (2001); Harborth et al., J. Cell Sci. 114:4557-65 (2001)). RNAi is mediated by double-stranded polynucleotides as also described hereinbelow, for example, double-stranded RNA (dsRNA), having sequences that correspond to exonic sequences encoding portions of the polypeptides for which expression is compromised. RNAi reportedly is not effected by double-stranded RNA polynucleotides that share sequence identity with intronic or promoter sequences (Elbashir et al., 2001). RNAi pathways have been best characterized in Drosophila and Caenorhabditis elegans, but “small interfering RNA” (siRNA) polynucleotides that interfere with expression of specific polynucleotides in higher eukaryotes such as mammals (including humans) have also been considered (e.g., Tuschl, 2001 Chembiochem. 2:239-245; Sharp, 2001 Genes Dev. 15:485; Bernstein et al., 2001 RNA 7:1509; Zamore, 2002 Science 296:1265; Plasterk, 2002 Science 296:1263; Zamore 2001 Nat. Struct. Biol. 8:746; Matzke et al., 2001 Science 293:1080; Scadden et al., 2001 EMBO Rep. 2:1107).
According to a current non-limiting model, the RNAi pathway is initiated by ATP-dependent, cleavage of long dsRNA into double-stranded fragments of about 18-27 (e.g., 19, 20, 21, 22, 23, 24, 25, 26, etc.) nucleotide base pairs in length, called small interfering RNAs (siRNAs) (see review by Hutvagner et al., Curr. Opin. Gen. Dev. 12:225-32 (2002); Elbashir et al., 2001; Nyknen et al., Cell 107:309-21 (2001); Zarnore et al., Cell 101:25-33 (2000)). In Drosophila, an enzyme known as “Dicer” cleaves the longer double-stranded RNA into siRNAs; Dicer belongs to the RNase III family of dsRNA-specific endonucleases (WO 01/68836; Bernstein et al., Nature 409:363-66 (2001)). Further, according to this non-limiting model, the siRNA duplexes are incorporated into a protein complex, followed by ATP-dependent unwinding of the siRNA, which then generates an active RNA-induced silencing complex (RISC) (WO 01/68836). The complex recognizes and cleaves a target RNA that is complementary to the guide strand of the siRNA, thus interfering with expression of a specific protein (Hutvagner et al., supra).
In C. elegans and Drosophila, RNAi may be mediated by long double-stranded RNA polynucleotides (WO 99/32619; WO 01/75164; Fire et al., 1998; Clemens et al., Proc. Natl. Acad. Sci. USA 97:6499-6503 (2000); Kisielow et al., Biochem. J. 363:1-5 (2002); see also WO 01/92513 (RNAi-mediated silencing in yeast)). In mammalian cells, however, transfection with long dsRNA polynucleotides (i.e., greater than 30 base pairs) leads to activation of a non-specific sequence response that globally blocks the initiation of protein synthesis and causes mRNA degradation (Bass, Nature 411:428-29 (2001)). Transfection of human and other mammalian cells with double-stranded RNAs of about 18-27 nucleotide base pairs in length interferes in a sequence-specific manner with expression of particular polypeptides encoded by messenger RNAs (mRNA) containing corresponding nucleotide sequences (WO 01/75164; Elbashir et al., 2001; Elbashir et al., Genes Dev. 15:188-200 (2001)); Harborth et al., J. Cell Sci. 114:4557-65 (2001); Carthew et al., Curr. Opin. Cell Biol. 13:244-48 (2001); Mailand et al., Nature Cell Biol. Advance Online Publication (Mar. 18, 2002); Mailand et al. 2002 Nature Cell Biol. 4:317).
siRNA polynucleotides may offer certain advantages over other polynucleotides known to the art for use in sequence-specific alteration or modulation of gene expression to yield altered levels of an encoded polypeptide product. These advantages include lower effective siRNA polynucleotide concentrations, enhanced siRNA polynucleotide stability, and shorter siRNA polynucleotide oligonucleotide lengths relative to such other polynucleotides (e.g., antisense, ribozyme or triplex polynucleotides). By way of a brief background, “antisense” polynucleotides bind in a sequence-specific manner to target nucleic acids, such as mRNA or DNA, to prevent transcription of DNA or translation of the mRNA (see, e.g., U.S. Pat. Nos. 5,168,053; 5,190,931; 5,135,917; 5,087,617; see also, e.g., Clusel et al., 1993 Nuc. Acids Res. 21:3405-11, describing “dumbbell” antisense oligonucleotides). “Ribozyme” polynucleotides can be targeted to any RNA transcript and are capable of catalytically cleaving such transcripts, thus impairing translation of mRNA (see, e.g., U.S. Pat. Nos. 5,272,262; 5,144,019; and 5,168,053, 5,180,818, 5,116,742 and 5,093,246; U.S. Ser. No. 2002/193579). “Triplex” DNA molecules refers to single DNA strands that bind duplex DNA to form a colinear triplex molecule, thereby preventing transcription (see, e.g., U.S. Pat. No. 5,176,996, describing methods for making synthetic oligonucleotides that bind to target sites on duplex DNA). Such triple-stranded structures are unstable and form only transiently under physiological conditions. Because single-stranded polynucleotides do not readily diffuse into cells and are therefore susceptible to nuclease digestion, development of single-stranded DNA for antisense or triplex technologies often requires chemically modified nucleotides to improve stability and absorption by cells. siRNAs, by contrast, are readily taken up by intact cells, are effective at interfering with the expression of specific polynucleotides at concentrations that are several orders of magnitude lower than those required for either antisense or ribozyme polynucleotides, and do not require the use of chemically modified nucleotides.